Communication services providers are subject to very strict bandwidth usage spectrum constraints, such as technically mandated specifications and regulations imposed by the Federal Communications Commission (FCC), which currently requires that sideband spillage, namely the amount of energy spillover outside a licensed band of interest, be sharply attenuated (e.g., on the order of 50 dB). While such limitations are adequate for traditional forms of modulation such as FM, they are difficult to achieve using more contemporary, digitally based modulation formats, such as M-ary modulation. To keep the sidebands attenuated sufficiently so as to meet industry or regulatory-based requirements using such modulation techniques dictates the use of very linear signal processing systems and components. Although linear components can be implemented at a reasonable cost at relatively low bandwidths (baseband) used in telephone networks, linearizing such components, in particular power amplifiers, at RF frequencies becomes a very costly exercise.
RF power amplifiers are inherently non-linear devices, and generate unwanted intermodulation products, which manifest themselves as spurious signals in the amplified output signal, separate and distinct from the input signal. A further source of RF amplifier distortion is the presence of spectral regrowth or spreading of a compact spectrum into spectral regions that do not appear in the input signal. The distortion introduced by an RF amplifier causes the phase and amplitude of its amplified output signal to depart from the respective phase and amplitude of the input signal, and may be considered as an incidental (and undesired) amplifier-sourced modulation of the input signal.
One brute force technique for linearizing an RF power amplifier is to build the amplifier as large, high power device and then operate the amplifier at a low power level that is only a small percentage of its rated output power, where the RF amplifier's transfer function is relatively linear. The obvious drawback to this approach is the overkill penalty--high cost and large size.
Other prior art attempts to account for RF amplifier degradation have included the installation of a `post-processing`, feed-forward correction loop that is coupled with the amplifier's output signal, and a `pre-processing`, or `predistortion` correction loop coupled in the path of the amplifier's input signal. The purpose of a post-processing, feed-forward correction loop is to extract the amount of error (distortion) present in the RF amplifier's output signal, amplify that extracted distortion signal to the proper level, and then reinject (the complement of) the amplified error signal back into the output path of the amplifier, such that (ideally) the amplifier distortion is effectively canceled.
A predistortion mechanism, on the other hand, serves to inject a `predistortion` signal into the RF amplifier's input signal path. The predistortion signal has a characteristic that has been predetermined to be ideally equal and opposite to the distortion expected at the output of the high power RF amplifier, so that when subjected to the transfer function of the RF amplifier, it will effectively cancel its anticipated distortion behavior. The predistortion mechanism may be made adaptive by extracting the error signal component in the output of the RF amplifier and adjusting the predistortion signal in accordance with the such extracted error behavior of the RF amplifier during real time operation, so as to effectively continuously minimize distortion in the amplifier's output.
In the past, such correction mechanisms have been implemented using discrete analog components, which are not capable of precisely tracking and compensating the highly non-linear behavior of the RF amplifier, so that they provide only a limited degree of linearization. This less than ideal performance of such components limits the linearity requirements that can be imposed upon cellular and PCS communications bands and thereby reduces bandwidth availability.